Field of the Disclosure
The present disclosure generally relates to apparatus and methods to directly control hydraulically-operated valves from remote locations. More particularly, the present disclosure relates to apparatus and methods to directly control devices operated by hydraulically-operated valves from remote locations over great distances at rapid speed and reduced latency. More particularly still, the present disclosure relates to apparatus and methods to directly control multiple subsea oilfield devices using hydraulically-operated valves at high speed from remote monitoring locations.
Description of the Related Art
In land-based oilfield production systems, retrieval and storage of the produced hydrocarbons (either in liquid or gaseous form) is relatively simple because surface tanks, pipelines, and valves may be constructed, filled, emptied, operated and monitored in-situ or remotely with relative ease. However, for subsea production systems, these tasks may be complicated by fact that production “trees” may be positioned atop subsea wellheads located beneath hundreds or thousands of feet of water upon the sea floor. Because of their location and large number, such installations may not be cost effectively monitored and manipulated by hand (either through the use of divers or remotely operated vehicles, or “ROVs”). As a result, hydraulic control systems may be used to monitor and control the various valves of a subsea tree installation from a remote location, either on-shore or at a centralized off-shore production facility (e.g., a platform or a ship).
In any given subsea production tree system, a plurality of valves may be used to control the production (and safety) of the hydrocarbons being produced. The most common types of hydraulically-operable valves used in hydrocarbon production are shutoff valves and choke (i.e., regulator) valves. Shutoff valves generally operate between two positions, fully open and fully closed and are often used as safety valves for rapidly closing off or “shutting in” the well either for routine maintenance or in the event of an emergency such as a hurricane or other weather event. In contrast, choke valves operate to regulate or vary the amount and flow of the hydrocarbons being produced from the subterranean or subsea wellbore. For example, as a reservoir is produced, the pressure may drop over a period of time. In such case, a choke valve may need to be set to be more restrictive of the flow of hydrocarbons in the early days of production than in the later days of production. Additionally, in certain circumstances, particularly when so-called 4-D fluid modeling is used, it may be advantageous to one wellhead to temporarily increase or restrict flow at a neighboring wellhead. Thus, the control of such “on-off” shutoff valves and their choke valve counterparts at a remote location is desirable throughout the life of the well.
For most installations, shutoff valves are designed to be maintained in an open position by hydraulic signal pressure and spring biased to default to the closed position should the signal pressure be lost. This configuration is based on the premise that should communication with the remote location be lost, it is safest for the valves to close and shut off the production of fluids. Conversely, choke valves may be operated through a series of hydraulic pulses sent from the remote location to instruct the choke valve in adjusting the regulation of produced fluids. In one example, an open/close choke operation valve may be “pulsed” open and closed to send control instructions to the hydraulic choke. Such arrangement may be referred to as a “stepped actuator” choke whereby the choke valve is configured to open or close a set increment for each open/close stroke received from the choke operation valve in communication therewith. Because the choke valve may be operated in two directions (i.e., opening and closing), a first stepped actuator may be used to operate the choke in an opening direction while a second stepped actuator may be used to operate the choke in a closing direction. Thus, similar valves may be used in both shutoff and variable choke applications using hydraulic remote control.
Generally, there are various different types of hydraulic control systems presently in use for remote control of subsea production tree valves. The first and least expensive to operate involves the control of each hydraulic valve directly from a remote location. In such “direct” systems, a single hydraulic conduit extends from each valve of the subsea tree to be operated to the remote location through a bundle of conduits commonly referred to as an “umbilical.” While these direct control systems are relatively simple and low-cost, they can be limited by the size and length of the umbilical needed. In particular, because each valve must be directly operated by its own conduit, the size (i.e., diameter) of the umbilical may increase to an unwieldy size if a production tree requires too many valves. Additionally, for applications where the umbilical must span large distances (e.g., over 1.5 km) between the production tree and the remote location, the time it takes to send pressure signals between the two locations may be negatively impacted by hydraulic friction (i.e., the viscosity of the hydraulic fluid) against the inner surface area of the conduit. While it may be possible to increase the diameter of the control conduits for such long distances, doing so may make the size of the overall umbilical excessively large or prohibitively expensive. Because there are regulatory restrictions regarding the minimum amount of time a subsea production shutoff valve may take to close once instructed, the use of such direct control systems may be limited by the distance between the valves to be operated and the remote location controlling them.
In the alternative, a multiplex or “MUX” hydraulic control system may be used. In a MUX installation, many hydraulically-operated devices may be controlled by a single hydraulic conduit extending from the wellhead to the remote location. A series of digitized or “multiplexed” pulses may be transmitted from the remote location where they are de-multiplexed or “de-MUXed” at the wellhead and separated into their individual signal conduits for each hydraulic device. Because the umbilical of a MUX installation may only contain a single hydraulic signal, the conduit may be sized sufficient to reduce any latency in the transmission that might otherwise result in a direct hydraulic control system of similar length/depth. However, while MUX systems are highly effective, they are relatively more expensive, as a significant amount of engineering must be invested to modulate and demodulate the hydraulic pulses into their component signals. Because of their expense, MUX systems are frequently used in deep water drilling and production environments where relative inaccessibility of the wellhead and the anticipated amount of production fluids are significant enough to justify their expense.
Thus, a control system capable of directly (and quickly) controlling subsea hydraulic valves over large distances would be highly desirable.